1. Field of the Invention
The present invention relates to a plate valve disposed in the midway of a flow passage, and, more particularly, to a plate valve which has shut-off and regulating functions working either in normal or in emergency state.
2. Description of the Related Art
A diversity of valve types disposed in the midway of a flow passage are known. One of the types is a globe valve which has typically a spherical casing, wherein the center line of its inlet passage is aligned with the center line of its outlet passage, and the flow of the fluid is curved in an S-shaped figure. The globe valve is a valve type that limits the fluid flow by checking directly the fluid flow. The advantage of this valve is that with its reliable choking capability, the valve regulates accurately the fluid flow, and that a compact design is easy to implement. The disadvantages of the globe valve: because of its construction, the valve needs a large handling torque, and thus a large size valve is not practicable; and a resulting flow resistance is large.
Another type of valve is a sluice valve (gate valve) which shuts off the fluid flow in a piping system. In this valve, a disk-like valve element closes the flow passage to shut off the fluid flow. The sluice valve is a valve for an on-off valving operation. The advantages of this valve: When it is fully opened, flow resistance is extremely small; a large diameter valve can be manufactured; and the handling torque for open and close operations is smaller than that for the globe valve. The disadvantages of the sluice valve are as follows: in its partially opened state, the valve suffers from eddied flow behind the valve element, giving rise to a large flow resistance and further causing the valve element to vibrate and then wear. The sluice valve thus is not appropriate to regulation of fluid flow. Furthermore, in its construction, the sluice valve has a long handling stroke for opening and closing operations, leading to a long opening or closing time involved. The sluice valve is thus not suitable for quick opening or closing valving application. Its small-sized versions are difficult to manufacture.
Another type of valve is a check valve. The check valve limits reverse flow from the downstream to the upstream of a flow passage because of back pressure. It automatically and quickly blocks reverse flow.
These three valves have their own advantages and disadvantages as described above. There has not been available the valve which offers functions and advantages of these valve combined.